Is there not an FAA office at Gatwick? Can you not use this to get your licence all sorted before you go.
I had a problem when I went to USA last summer. The flight school where I wanted to rent a plane said my paper licence was not valid (had it for 22 years issued on my CAA PPL). They said I needed a plasic one. In the end it was accepted, but they were a real pain. They did releive me of a biannual and check out of course before saying I could not fly - I am a UK flying instructor and have flown about 300 hours (5000 total time) in the USA and they still did a 2 hour flight check and 2 hours ground school (usual rip off). it took me 4 days to get it ok'd by the FAA. Of cousre it was an Orlando flight school!!! Having said all that great flying in a nearly brand new C172